1. Field of the Invention
The present invention relates to an organic electroluminescent element array (hereinafter referred to as an organic EL element array) including a plurality of organic electroluminescent elements (hereinafter referred to as organic EL elements).
2. Description of the Related Art
In recent years, organic EL elements have been widely investigated. The organic EL elements each typically include an anode, a cathode, and an organic compound layer which is disposed between the anode and the cathode and which includes a light-emitting sublayer. Holes and electrons which are injected into the organic compound layer by applying a voltage between the anode and the cathode are recombined with each other in the light-emitting sublayer, whereby light is emitted from the light-emitting sublayer. An array of the organic EL elements can be used as a display. In particular, a full-color display can be formed by arraying organic EL elements emitting light of different colors.
Since current organic EL elements lack desired light-emitting efficiency, they need to be improved in their light-emitting efficiency. One way to improve light-emitting efficiency is to use constructive optical interference.
International Publication WO 01/039554 (hereinafter referred to as Patent Document 1) discloses an organic EL element having a resonant structure consisting of a first electrode made of a light-reflective material, an organic layer serving as a resonant section, and a translucent reflective layer. Patent Document 1 also discloses an organic EL element including a first electrode made of a light-reflective material and serving as a cathode; a second electrode made of a transparent material and serving as an anode; and an organic layer that forms a resonant section together with the second electrode. In this organic EL element light emitted from the organic layer is reflected between the lower end of the organic layer (that is, the interface between the organic layer and the first electrode) and the upper end of the second electrode (that is, the interface between the second electrode and air).
As disclosed in Patent Document 1, the optical path of each resonant section is adjusted to a positive minimum value such that the organic EL elements satisfy the following equation:2L/λ+Φ/2Π=m wherein L represents the optical path of the resonant section, λ represents the wavelength of light extracted from the organic EL element, Φ represents the sum of the phase shifts at both ends of the resonant section, and m represents a positive integer.Since the optical path thereof is adjusted to such a positive minimum value, the chromaticity of the organic EL element can be improved over a wide range of a view angle.
When that the organic EL elements disclosed in Patent Document 1 satisfy the above equation, the optical path of each resonant section has a positive minimum value. Hence, m, which is the order of interference, is equal to one.
When the optical path thereof has a positive minimum value, each organic layer has a minimum thickness. A reduction in the wavelength of light emitted from each organic EL element leads to a reduction in the thickness of the organic layer. Furthermore, an increase in the number of sublayers in the organic layer, which is included in the resonant section, leads to a reduction in the thickness of each sublayer.
An excessive reduction in the thickness of the organic layer prevents formation of the sublayers having uniform thickness. This causes problems such as short circuits, current leakage and/or a problem in that the ability to transport charges and/or the ability to emit light is impaired. In organic EL element arrays including three types of elements; that is, red light-emitting elements (hereinafter referred to as R elements), green light-emitting elements (hereinafter referred to as G elements), and blue light-emitting elements (hereinafter referred to as B elements), the above problems are likely to occur in the B or G elements.
If the thickness of the organic layer is increased such that m is greater than or equal to two, then the problems of impaired ability to transport charges and/or impaired ability to emit light can be prevented. However, an increase in the thickness of the organic layer may cause corresponding problems in that voltage for driving the organic EL element is increased and/or the view angle is reduced (as disclosed in Patent Document 1) because the order of interference is high.